1. Field of the Invention
This invention relates to a glass-based optical waveguide.
2. Description of the Related Art
An optical waveguide having a core layer and a clad layer formed on a substrate is manufactured by a thin-film technology or a fine processing technology similar to a semiconductor process, and is drawing attention as a principal component in a high-speed optical fiber network. Since the glass-based optical waveguide has low loss and is easy to manufacture, the waveguide is occupying a majority proportion of the optical waveguide devices which have been placed in the market as products.
An optical fiber is connected to an input terminal of the optical waveguide. The state of polarization of input light from the optical fiber is indefinite. Therefore, it is desired that the characteristics of the optical waveguide do not vary depending upon the state of polarization of the input light. The polarization dependence of the optical waveguide is caused by the birefringence. The birefringence stems from the internal stress built up in the core layer and the clad layer, and is particularly greatly affected by thermal stress stemming from differences in the coefficients of linear expansion of the layers. In a process for manufacturing a silica waveguide, in general, the heat treatment is conducted after film formation in order to decrease the insertion loss of the manufactured optical waveguide. In a step in which the temperature is lowering after the heat treatment, the thermal stress occurs from a difference in the coefficients of linear expansion among the core layer, the clad layer and the substrate. When a difference in the thermal stress occurs between the in-plane direction of the substrate and the direction perpendicular thereto near the optical waveguide, the birefringence occurs in the optical waveguide due to photoelastic effect.
In order to solve the above problem, there have been proposed, for example, a method of equalizing the coefficients of linear expansion of the core layer and the clad layer on the interface between the layers (see JP-A-1-169406), a method of equalizing the coefficients of linear expansion of the upper clad layer and the substrate (see JP-A-11-174246), and a method of optimizing the amount of dopant such that the coefficients of linear expansion of the clad layer and the core layer are equalized (see JP-A-5-257021). Even by employing the above known methods of equalizing the coefficients of linear expansion of layers on the interface between the layers, however, it is not still possible to obtain an optical waveguide which has sufficient polarization independence.